US Northern Command has placed an operational hold on all military flights of Modular Airborne Fire Fighting System-equipped Lockheed Martin C-130 aircraft after a 1 July crash.

The downed aircraft belonged to the North Carolina Air National Guard's 145th Airlift Wing. The crew was helping to fight a wild fire in South Dakota when it crashed at around 18:30 local time. It was the first crash of a C-130 so-equipped in over 40 years.

The crash is now under investigation, Northern Command says.

"There were casualties, and our thoughts and prayers go out to those who were injured and those who lost their lives," Northern Command says. "The family members of these airmen are especially on our minds. We will provide further details on the status of the casualties soon."

President Barrack Obama also expressed his condolences and offered his thanks to the aircrews.

"I know that Americans join me in expressing my deepest gratitude for the selfless determination they and thousands of men and women involved in this fight in states across the country demonstrate every day," Obama says. "I know Americans across the country share my concern for the well-being of the surviving members of the crew and my deep condolences to the families of those who lost their lives."

He adds: "They are heroes who deserve the appreciation of a grateful nation."